I'm just grasping at another logic, sorry for the answer.
dragon V dragon
It's not hard to imagine why most types are resistant to themselves, like two physics locked into an intense mental battle or two fish just splashing water into one another's faces. But imagine an awesome dragon war in the sky? One word. EPIC. That's why Dragon V Dragon is super effective.
ghost V ghost
Even some ghosts might be merciful people, I think most of them as evil people destined to roam the world forever, unable to meet their friends and family in heaven as a punishment for evil deeds. I think them as in a reverse world, where each overpowers but are in the same strength.
Something like that.